Polyester fibers are excellent in mechanical properties, dimensional stability and endurance, and therefore, they have been widely used not only for clothing applications but also for industrial applications, particularly for rubber reinforcement of industrial materials such as tire cords, V-belts, conveyor belts and hoses. For tire cord applications, the use of radial tires for automobiles has been advanced, so that there have been requested comfortable able feeling and excellent driving stability in the high-speed running as well as lightweightness for the reduction of fuel consumption. Therefore, fibers having high modulus and low shrinkage and further having high strength have been strongly requested as dipped cords for carcass plies.
For the preparation of polyethylene terephthalate fibers having these excellent characteristics, there has been known a process comprising melt spinning polyethylene terephthalate, followed by taking up at a relatively high spinning speed of 1000 to 3000 m/min to give a high oriented undrawn yarn having a birefringence of 0.02 to 0.07, what is called POY, and then heat drawing the POY at a low ratio of 1.5 to 3.5 (hereinafter referred to as the POY method). The polyester fibers prepared by such a process are very excellent as a technique of attaining high modulus and low shrinkage, in comparison with high strength fibers prepared by a process comprising melt spinning, followed by taking up at a low spinning speed of 1000 m/min or lower to give a low oriented undrawn yarn having a birefringence of 0.01 or lower, and then heat drawing the undrawn yarn at a high ratio of 4 to 7 (hereinafter referred to as the UDY method).
However, the polyester fibers obtained by the POY method, although they have excellent characteristics as described above, have apparently lower strength and breaking elongation than the polyester fibers obtained by the UDY method, and when the fibers have low breaking elongation in such a manner, a decrease of tenacity becomes great in the twisting and in the dip treatment, so that the resulting cords have low tenacity which is insufficient as dipped cords.
To comply with such a request, for example, in the Japanese Patent No. 2,569,720, high strength dipped cords exhibiting excellent tenacity conversion efficiency in the dip treatment are obtained by the use of a base yarn meeting the conditions that breaking elongation≧11% and toughness is 30 to 36 g/d. %0.5 (26.5 to 31.8 cN/dtex. %0.5). For dimensional stability, however, these dipped cords cannot meet the level further requested in recent years for high modulus and low shrinkage, by which substitution for rayon may be taken into account.
In the Japanese Patent No. 2,775,923, high strength dipped cords are obtained by the use of a base yarn meeting the condition that toughness≧elongation at a specific load+dry heat shrinkage+22.0. However, the dimensional stability of dipped cords is not expressly described in this patent publication, the level of which is therefore unknown.